(CNN) -- The National Transportation Safety Board will assist the South Korean government in investigating the crash of an Asiana Airlines cargo plane, officials said Friday.

Asiana Flight 991, a Boeing 747-400F cargo airplane, went down Thursday near the southern South Korean island of Jeju, where parts of the plane were found but not the aircraft itself.

The two pilots are believed to have been killed, the safety board said in a statement.

The plane crashed around 4:12 a.m. en route from Seoul's Incheon International Airport to Shanghai Pudong International Airport in China, the NTSB said.

The flight crew reportedly declared an emergency due to an in-flight fire, the board said. The plane crashed into the East China Sea about 70 miles west of Jeju Island, the NTSB said.

U.S. officials have designated air safety investigator John Lovell as the NTSB representative, to be joined by an NTSB operational factors investigator and advisors from the Federal Aviation Administration and Boeing, the safety board said.

The Korea Aviation and Railway Accident Investigation Board is leading the probe, the U.S. safety board said.